Evolutionary Change: A Case Study of Extinct Brachiopod Species

Abstract

Brachiopods serve as great tools for quantifying evolution of early life. In this study, morphological characters were quantified for 1,100 individual specimens of a single atrypid species from the Middle Devonian Traverse Group of Michigan using geometric morphometric methods. Seven landmark measurements were taken on dorsal valve, ventral valve, and anterior and posterior regions. Specimens were partitioned by their occurrence in four stratigraphic horizons (Bell Shale, Ferron Point, Genshaw Formation, and Norway Point) from the Traverse Group of northeastern Michigan outcrop. Multivariate statistical analyses were performed to test patterns and processes of morphological shape change of species over 6 million year interval of time. Maximum-likelihood method was used to determine the evolutionary rate and mode in morphological divergence in this species over time. If punctuated equilibrium model holds true for brachiopods, then one would expect no significant differences between samples of the species Pseudoatrypa cf. lineata from successive stratigraphic units of the 6 million year Middle Devonian Traverse Group over time. Multivariate analysis shows significant shape differences between different time horizons (p ≤ 0.01) with considerable overlap in morphology excepting abrupt deviation in morphology in the uppermost occurrence. Maximum-likelihood tests further confirm near stasis to near random divergence mode of evolution with slow to moderate rates of evolution for this species lineage. In contrast, if the species evolved in a gradual, directional manner, then one would expect samples close together in time to be more similar to one another than those more separated in time. Euclidean based cluster analysis shows samples closely spaced in time are more similar than those that are widely separated in time. While these results appear to partially support the gradualistic model hypothesis, morphological trend from principal component scores shows substantial morphological overlap among the three lower successions (Bell Shale, Ferron Point, and Genshaw Formation) with some deviation from the uppermost succession (Norway Point) suggesting major influence of stasis and punctuation. Thus, while stasis may have been predominant, evident from stable morphologies observed in the lower strata, anagenetic evolution may also have played an important role as evident from the abrupt change observed within this species later in time. Thus, slow to moderate rates of evolution in this species lineage with stable morphologies in the lower three strata supports stasis, but abrupt change in the uppermost strata supports gradual anagenetic evolution within the species later in time. Overall, the morphometric data for the P. cf. lineata species lineage are consistent with the stratigraphic succession of Traverse Group.